Image forming apparatus

ABSTRACT

In a scanning optical system provided with a plurality of scanning optical devices, each scanning optical device including a light source to emit a light beam, a collimator lens into which the light beam emitted from the light source enters, a deflector to deflect the light beam passing through the collimator lens in a main scanning direction, an image forming lens to focus the light beam coming from the deflector onto a scanned surface, and a base plate on which the light source, the collimator lens, the deflector and the image forming lens are fixed, the plurality of scanning optical devices are piled up in a sub-scanning direction perpendicular to the main scanning direction in such a manner that each base plate is substantially parallel to other base plates; and the scanning optical system is further provide with a plurality of link members to connect two neighboring base plates independently of other base plates.

This is a Continuation of application Ser. No. 09/406,244 filed Sep. 24,1999 now U.S. Pat. No. 6,512,533.

BACKGROUND OF THE INVENTION

This invention relates to a scanning optical device comprising at leasta light source, a collimator lens into which a light beam emitted fromsaid light source enters, a deflector which deflects the light beamhaving passed said collimator lens to the direction of main scanning,and an image forming lens which focuses the light beam having passedsaid deflector on the surface to be scanned and to an image formingapparatus having a plurality of the above-mentioned scanning opticaldevices arranged in the direction of sub-scanning.

Generally speaking, in an image forming apparatus capable of forming amulti-color image, a cylinder-shaped, or belt-shaped image bearingmember is used. In forming a multi-color image by using this imagebearing member, charging, exposure and development are carried out foreach of colors with the image bearing member rotated (moved), and themultiple color toner images are superposed one after another on theimage bearing member and transferred onto a sheet of transfer paper by aone-time transfer operation.

Incidentally, in order to accomplish a high-speed printing, it isrequired not a structure such that exposure processing (writing bylight) for one color is carried out in every rotation of the imagebearing member but a structure such that the exposures for all of fourcolors Y, M, C, and K are carried out in every rotation of the imagebearing member. In this case, not only a plurality of the developingunits but also a plurality of the scanning optical devices correspondingto the respective colors should be provided in order that the multipleexposures and developments should be done at different positions on theimage bearing member.

For example, in the case where a color image is formed using the fourcolors Y (yellow), M (magenta), C (cyan), and K (black), the scanningoptical devices and the developing units for the respective colors Y, M,C, and K are arranged in the direction of sub-scanning, and each of thescanning optical devices is let to form the latent electrostatic imageof the color corresponding to it in such a manner as to form the tonerimages precisely superposed on one another, and the latent electrostaticimages are developed.

In this case, to remark one pixel of the latent images formed by therespective scanning optical devices, if the position of this pixel isdeviated for a certain color, synthesis of the color can not be done atthis pixel portion, producing a color deviation and lowering resolution,and image quality is deteriorated. In order to prevent the occurrence ofthis color deviation and the lowering of resolution, it is required thatnot only the characteristics of the respective scanning optical devicesare made to be the same so as to be able to write the same scan line,but also the respective optical devices are fixed in a precisepositional relationship so as to make the scan lines for the respectivecolors coincide with one another.

To consider the characteristics of the scanning optical device as anindividual one, it is desirable for obtaining a good image that the scanlines on the surface to be scanned are straight and, at the same time,the arrangement of the pixels forming any one of the scan lines has auniform interval between any adjacent pixels. However, actually, thepositioning of optical parts etc can not be precisely performed,therefore, there have been problems that scan lines on the surface to bescanned are curved and that the arrangement of the pixels forming a scanline has an uneven interval between pixels.

Further, in the case where a multi-color image is formed, there has beena problem that the characteristics of the respective scanning opticaldevices can not be made equal, neither the respective scanning opticaldevices can be fixed in a precise positional relationship, resulting indeterioration of image quality.

SUMMARY OF THE INVENTION

This invention has been made in order to solve the above-describedproblems, and it is an object of this invention concerning a scanningoptical device to actualize a scanning optical device capable ofadjusting the scanning beam easily.

Further, it is an object of this invention concerning an image formingapparatus to actualize an image forming apparatus capable of getting ridof the deterioration of image quality owing to the scanning opticaldevice.

The above object can be attained by the following structures.

A scanning optical system, comprises:

a plurality of scanning optical devices, each scanning optical deviceincluding

a light source to emit a light beam,

a collimator lens into which the light beam emitted from the lightsource enters,

a deflector to deflect the light beam passing through the collimatorlens in a main scanning direction,

an image forming lens to focus the light beam coming from the deflectoronto a scanned surface, and

a base plate on which the light source, the collimator lens, thedeflector and the image forming lens are fixed, wherein the plurality ofscanning optical devices are piled up in a sub-scanning directionperpendicular to the main scanning direction in such a manner that eachbase plate is substantially parallel to other base plates; and

a plurality of link members to connect two neighboring base platesindependently of other base plates.

A scanning optical system, comprises:

a plurality of scanning optical devices, each scanning optical deviceincluding

a light source to emit a light beam,

a collimator lens into which the light beam emitted from the lightsource enters,

a deflector to deflect the light beam passing through the collimatorlens in a main scanning direction,

an image forming lens to focus the light beam coming from the deflectoronto a scanned surface,

a contact member to come in contact with a first side surface of theimage forming lens at three contact points on both end portions and amiddle portion of the first side surface in terms of the main scanningdirection;

a pressing member to come in pressure contact with a second side surfaceof the image forming lens so as to press the image forming lens towardthe contact member and

a base plate on which the light source, the collimator lens, thedeflector, the image forming lens, the contact member and the pressingmember are fixed, wherein the plurality of scanning optical devices arepiled up in a sub-scanning direction perpendicular to the main scanningdirection in such a manner that each base plate is substantiallyparallel to other base plates; and

a plurality of link members to connect two neighboring base platesindependently of other base plates.

A scanning optical device, comprises:

a light source to emit a light beam;

a collimator lens into which the light beam emitted from the lightsource enters;

a deflector to deflect the light beam passing through the collimatorlens in a main scanning direction;

an image forming lens to focus the light beam coming from the deflectoronto a scanned surface;

a contact member to come in contact with a first side surface of theimage forming lens at three contact points on both end portions and amiddle portion of the first side surface in terms of the main scanningdirection; and

a pressing member to come in pressure contact with a second side surfaceof the image forming lens so as to press the image forming lens towardthe contact member.

Further, the above object can be attained by the following preferablestructures.

This invention concerning a scanning optical device is the onecomprising at least a light source, a collimator lens into which a lightbeam emitted from said light source enters, a deflector which deflectsthe light beam having passed said collimator lens to the direction ofmain scanning, and an image forming lens which focuses the light beamhaving passed said deflector on the surface to be scanned, said scanningoptical device further comprising contact members capable of being incontact with said image forming lens at three points, that is, the bothend portions of said image forming lens in the direction of mainscanning and an intermediate point between the both end pointsrespectively at the time of adjusting the position of said image forminglens, and an urging means for urging said image forming lens toward saidcontact members.

In this invention, a mechanism for adjusting and determining theposition of the image forming lens is provided. The image forming lensis positioned in the vicinity of the surface to be scanned, and is easyto have a bend owing to its long size. In spite of this, according tothis invention for adjusting the position to fix this image forminglens, the scanning beam is corrected easily and with a good efficiency.In addition to it, because the image forming lens is in contact with thecontact members at the three points, namely, the both end portions inthe main scanning direction and an intermediate portion between the bothend portions at the time of adjusting the position of the image forminglens, it is possible to let the image forming lens take any position bydisplacing the points of contact.

In this case, if the points of contact of the image forming lens at itsboth end portions in the main scanning direction with the contactmembers are made to be positioned on a straight line which isapproximately parallel to the main scanning direction, and the point ofcontact at the intermediate portion is placed at a position deviatedfrom a position on the above-mentioned straight line in the directionperpendicular to the surface to be scanned, the adjustment of positionof the image forming lens can be performed by moving at least one of thecontact members placed at the above-mentioned three points of contact inthe direction of sub-scanning.

It is desirable that the image forming lens is fixed to the base inorder to prevent the deviation of the position of the image forminglens, after the adjustment and determination of the position of theimage forming lens. By carrying out this fixing at a position in theneighborhood of the point of contact at the intermediate portion of theimage forming lens, the image forming lens is fixed only at theintermediate portion, which makes it possible for the image forming lensto expand and contract freely in accordance with the variation oftemperature and humidity of the surrounding; thus the image forming lensis never bent unnaturally and it can be avoided the unfavorableinfluence that a complex bending etc are produced in the scan lines.

This invention concerning an image forming apparatus is the one having aplurality of scanning optical devices arranged in the direction ofsub-scanning, each of said scanning optical devices comprising at leasta light source, a collimator lens into which a light beam emitted fromsaid light source enters, a deflector which deflects the light beamhaving passed said collimator lens to the direction of main scanning,and an image forming lens which focuses the light beam having passedsaid deflector on the surface to be scanned, wherein any pair of theneighboring scanning optical devices are linked to each other, and thelinking between these scanning optical devices is made by fixing a linkmember to each of the scanning optical devices in a manner capable oftaking off, the position of one of the neighboring scanning opticaldevices being determined in the state of being freely movable(hereinafter referred to as ‘state of floating off’) against the other,and after that, fixing said linking members of the neighboring scanningoptical devices to one another.

According to this invention, the positions of a plurality of scanningoptical devices arranged in the direction of sub-scanning are adjustedand determined in the state of floating off against the others, and thelinking members are mutually linked after determining the positions.Owing to this, by once precisely adjusting and determining the positionsof the respective scanning optical devices, the positional relationshipamong the scanning optical devices will never be varied after that, andthe deterioration of image quality owing to the scanning optical devicecan be prevented.

Further, because the linking members are fixed to the respectivescanning optical devices in a manner capable of being taken off, in thecase where the determination of the positions is proved to be notsatisfactory after the linking of the neighboring scanning opticaldevices, it is possible that these link members are taken off from thescanning optical devices, with the state of mutual linking of the linkmembers let to remain as it is, and then a new link members are attachedto the scanning optical devices, and the positions are determined again,and the new link members are fixed to one another. Owing to this, it hasbecome possible to utilize again the scanning optical devices.

Another invention concerning an image forming apparatus is the onehaving a plurality of scanning optical devices arranged in the directionof sub-scanning, each of said scanning optical devices comprising atleast a light source, a collimator lens into which a light beam emittedfrom said light source enters, a deflector which deflects the light beamhaving passed said collimator lens to the direction of main scanning,and an image forming lens which focuses the light beam having passedsaid deflector on the surface to be scanned, wherein each of saidscanning optical devices comprises contact members capable of being incontact with said image forming lens at three points, that is, the bothend portions of said image forming lens in the direction of mainscanning and an intermediate point between the both end pointsrespectively at the time of adjusting the position of said image forminglens, and an urging means for urging said image forming lens toward saidcontact member, and further, any pair of the neighboring scanningoptical device are linked to each other, and the linking between thesescanning optical device is made by fixing a link member to each of thescanning optical devices in a manner capable of taking off, the positionof one of the neighboring scanning optical devices being determined inthe state of floating off against the other, and after that, fixing saidlinking members of the neighboring scanning optical devices to oneanother.

According to this invention, in each of the scanning optical devices,the scanning beam can be corrected easily and with a good efficiency.Further, because the linking members are mutually linked afterdetermining the positions, the positional relationship among thescanning optical devices is never varied, and the deterioration of imagequality owing to the scanning optical device can be prevented.Furthermore, reutilization of the scanning optical device has becomepossible.

If an adhesive of the UV-hardening type is used for the above-describedfixing of the scanning optical devices to one another, the determinationof the positions is carried out with the adhesive injected into everyclearance between the linking members, and the fixing can be done byapplying an ultraviolet ray immediately after the determining of thepositions, which makes the linking operation easy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the plan showing an example of the embodiment of thisinvention concerning a scanning optical device;

FIG. 2 is the front view showing the example of the embodiment of thisinvention concerning a scanning optical device;

FIG. 3 is the bottom view showing the example of the embodiment of thisinvention concerning a scanning optical device;

FIG. 4 is the enlarged plan showing a part of the base;

FIG. 5 is the front view of the second cylindrical lens;

FIG. 6 is the plan of the second cylindrical lens;

FIG. 7 is a cross-sectional view along the cutting line A—A in FIG. 5;

FIG. 8 is a perspective view showing the structure for urging the secondcylindrical lens;

FIG. 9 is a drawing showing the structure of the scanning optical devicein the linked state;

FIG. 10 is a perspective view showing the structure of fitting a linkmember; and

FIG. 11 is a drawing showing the structure of the scanning opticaldevices in the linked state.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT THE FIRST EXAMPLE OFTHE EMBODIMENT

FIG. 1 to FIG. 3 are drawings for illustrating an example of theembodiment of this invention concerning a scanning optical device; FIG.1 is the plan, FIG. 2 is the front view, and FIG. 3 is the bottom view.

As shown in FIG. 1, the laser light source 2 emitting a laser beam inthe horizontal direction, the collimator lens 3, and the firstcylindrical lens 3′ are attached to the standing wall portion 11, and onthe bottom plate portion 12 of the base 1, there is provided thepolygonal mirror 4 as a deflector for deflecting the laser beam havingbeen emitted from the laser light source 2 and having passed thecollimator lens 3 in the direction of main scanning in such a manner asto be capable of rotating around a vertical axis. Further, on the bottomplate portion 12 of the base 1, there are attached the fθ lens 5 intowhich the laser beam reflected by the polygonal mirror 4 enters, and thesecond cylindrical lens 6 as an image forming lens for focusing thelaser beam having passed the fθ lens 5 on the surface to be scanned.

As shown in FIG. 4, at the mounting portion of the fθ lens 5 on thebottom plate portion 12 of the base 1, there are provided theprojections 13 and 14 which are in contact with the lower surface of theboth end portions of the fθ lens 5, the projection 15 which is incontact with the lower surface of the intermediate portion of the fθlens 5, the projections 16 and 17 which are in contact with the sidesurface of the both end portions of the fθ lens 5, and the projection 18for adhesive bonding which faces the lower surface of the intermediateportion of the fθ lens 5. The fθ lens 5 is bonded by an adhesive to theprojection 18 for adhesive bonding after it is set in the state ofcontact with the above-mentioned projections 13 to 17.

As shown in FIG. 5 to FIG. 7, the second cylindrical lens 6 has the rimportion 61 at its upper portion and the rim 62 at its lower portion, andthe portion between them is the effective lens portion 63. At the lowersurface of the both end portions of the rim portion 62, the supportingportions 65 and 66 are formed respectively, and also at the lowersurface of the intermediate portion of the rim portion 62, thesupporting portion 67 is formed. Further, the engaging projection 68 isformed on the supporting portion 67.

As shown in FIG. 1 and FIG. 4, at the mounting portion of the secondcylindrical lens 6 on the bottom plate portion 12 of the base 1, thereare provided the projections 19 and 20 which are capable of being incontact with the lower surface of the supporting portions 65 and 66 ofthe second cylindrical lens 6, the projections 22 and 23 which arecapable of being in contact with the side surface of the both endportions of the second cylindrical lens 6, and the projections foradhesive bonding 24 and 25 which face the lower surface of thesupporting portion 67 of the second cylindrical lens 6.

Further, at the positions in the neighborhood of the projections 19 to21 facing the supporting portions 65 to 67 respectively, the screw holes26 to 28 are bored. Further, in the neighborhood of the projections 22and 23, the supporting rods 29 and 30 are fitted as projected.

For mounting the second cylindrical lens 6 to the bottom plate portion12 of the base 1, first, the second cylindrical lens 6 is placed on theprojections 19 to 21 with the supporting portions 65 to 67 positioned atthe bottom. At this time, the side surfaces of the both end portions ofthe cylindrical lens 6 are supported by the projections 22 and 23, andthe engaging projection 68 of the second cylindrical lens 6 is fittedinto the slot 24 a (refer to FIG. 4) of the projection for adhesivebonding 24 on the bottom plate portion 12. By this fit, the secondcylindrical lens 6 is regulated with regard to the movement in the mainscanning direction.

Next, as shown in FIG. 1 and FIG. 8, the base portions of the leafsprings 31 and 32 as urging means are fixed to the supporting rods 29and 30 respectively by the screws 33 and 34, and the upper surfaces ofthe both end portions of the rim portion 61 of the cylindrical lens 6 ispressed downward by the free end portions of the leaf springs 31 and 32(that is, the half-sphere-shaped projections 31 a and 32 a provided asprojected downward in FIG. 8). This state of pressing is a temporaryfixed state by an elastic force, and if a force is exerted to the secondcylindrical lens 6, it can be removed.

In this example of the embodiment, for the purpose of performing theadjustment of the position of the second cylindrical lens 6, the screwsfor adjustment 36 to 38 as members for contact are forced to be insertedinto the screw holes 26 to 28 from the bottom side. The front ends ofthese screws for adjustment 36 to 38 are capable of being in contactwith the supporting portions 65 to 67 of the second cylindrical lens 6at least at the time of adjusting the position of the second cylindricallens 6, and if the front ends of the screws for adjustment 36 to 38 areprojected upward farther than the projections 19 to 21, they become incontact with the supporting portions 65 to 67 of the second cylindricallens 6. Because the second cylindrical lens 6 is always pressed by thehalf-sphere-shaped projections 31 a and 32 a of the leaf springs 31 and32 toward the screws for adjustment 36 to 38, it is moved as engagedwith the screws for adjustment 36 to 38.

In this example of the embodiment, the points of contact of the secondcylindrical lens 6 against the screws for adjustment 36 and 37 at theboth end portions in the main scanning direction are positioned on astraight line which is approximately parallel to the main scanningdirection, and the point of contact at the intermediate portion againstthe screw for adjustment 38 is deviated from a position on theabove-mentioned straight line in the direction perpendicular to thesurface to be scanned.

Accordingly, by rotating the screws for adjustment 36 to 38 separatelyto adjust the amount of projection, it is possible not only the parallelmovement of the second cylindrical lens 6 in the upward and downwarddirection (the direction of sub-scanning), but also its tilting movementin the upward and downward direction (rotation around an axis parallelto the direction of main scanning) or its tilting movement in the leftand right direction (rotation around an axis perpendicular to thesurface to be scanned); thus, the second cylindrical lens 6 can take anydesirable position. After this adjustment and determination of theposition for the second cylindrical lens 6, the rim portion 62 is bondedto the projections for adhesive bonding 24 and 25 by using an adhesiveof the UV-setting type or the like.

As shown in FIG. 2, in the front wall portion 40 of the base 1, which ispositioned in front of the second cylindrical lens 6 and faces thesurface to be scanned, it is bored the slit 41 for letting the laserbeam having passed the second cylindrical lens 6 emerge, and the frame42 is formed in such a manner as to surround this slit 41. Further, inthis frame 42, the transparent cover 43 is bonded in such a manner as toclose the slit 41.

In the neighborhood of one end portion of the second cylindrical lens 6,the mirror 7 is disposed in order that the laser beam having entered onit from the polygonal mirror 4 through the fθ lens 5 may be reflectedtoward the photo-sensor 8 which is attached to the standing wall portion11. This photo-sensor 8 is provided for detecting that the laser beamreaches the predetermined beam position, and its output is used for thesynchronization of main scanning.

The scanning by the laser beam in this example of the embodiment isquite the same as in the apparatus of the prior art. That is, the laserbeam having been emitted from the laser light source 2 to the horizontaldirection is made to be a parallel pencil of light by the collimatorlens 3, and after being regulated by a slit, which is not shown in thedrawings, it enters onto the polygonal mirror 4; then, the reflectedlaser beam by the polygonal mirror 4 passes the fθ lens 5, and entersinto the second cylindrical lens 6. Further, the laser beam havingpassed the second cylindrical lens 6 reaches the surface to be scannedthrough the slit 41.

In the above, because the polygonal mirror 4 is rotating, the beam spoton the surface to be scanned moves to the direction of main scanning,and an exposure process in accordance with the modulation in the laserlight source 2 is carried out. Further, because the surface to bescanned is moving in the direction of sub-scanning, the surface to bescanned is exposed to the laser beam two-dimensionally. The exposurestart timing on every scanning line is determined on the basis of thephoto-sensor 8.

In this example of the embodiment, a mechanism for adjusting anddetermining the position of the second cylindrical lens 6 is provided.The reason for it is that the second cylindrical lens 6 is positionedclose to the surface to be scanned, and is easy to produce bendingbecause it is a long-sized lens. According to this example of theembodiment wherein the mounting position of the second cylindrical lens6 is adjusted, the scanning beam is corrected easily and in a goodefficiency. In addition to it, because the second cylindrical lens 6 isin contact with the contact members 37 to 39 at three points, that is,the both end portions in the main scanning direction and theintermediate portion between these both end points respectively at thetime of adjusting and determining the position of the second cylindricallens 6, it is possible to let the second cylindrical lens 6 take anyposition by displacing the contact points.

Further, after adjusting and determining the position of the secondcylindrical lens 6, it is fixed to the base 1 in order to prevent thepositional deviation of the second cylindrical lens 6; this fixing isdone, in this example of the embodiment, in the neighborhood of thecontact point at the intermediate portion of the second cylindrical lens6. Owing to this, the second cylindrical lens 6 is fixed only at theintermediate portion, it can freely expand and contract in accordancewith the surrounding temperature and humidity, it is never bentunnaturally, and it can be avoided the unfavorable influence that acomplex bending etc are produced in the scan lines.

It will be explained in more detail the operation of adjusting anddetermining the position of the second cylindrical lens 6 in theabove-described example of the embodiment. In addition, here, theexplanation will be given on the condition that there is no tilt angleerror for each of the reflecting surfaces of the polygonal mirror 4.

The operation of adjusting and determining the position is carried outfor the scanning optical devices in the state wherein the secondcylindrical lens 6 is temporarily fixed. At this time, for the scan linedetecting portion of the fixture for adjustment, it is used the onewherein three sensors are arranged on an ideal scan line which isassumed to be on the surface to be scanned. This sensor is the onecapable of detecting to what degree an actual beam is deviated from theabove-mentioned ideal scan line, and any method of detection can beused.

It can be known what kind of a curve an actual scan line is tracing fromthe output of the three sensors when a laser beam actually scans thesurface. Therefore, in order that the curve as a whole may become closerto the ideal scan line, the amount of screwing for the screws foradjustment 37 to 39 is adjusted. The rotating operation of the screwsfor adjustment at this time is performed, for example, as follows:

(1) Parallel movement in the upward and downward direction (thedirection of sub-scanning)

All the screws for adjustment 37 to 39 are rotated in the samedirection.

(2) Tilting movement in the upward and downward direction (Rotationaround an axis parallel to the main scanning direction)

The screws for adjustment 37 and 38 are rotated in the same direction,or the screw for adjustment 39 is rotated.

(3) Tilting movement in the left and right direction (Rotation around anaxis perpendicular to the surface to be scanned)

The screw for adjustment 37 or 38 is rotated.

(4) Correction (for example, correction for bending)

This is possible in the case where the second cylindrical lens 6 iscapable of bending like a plastic lens; the screws for adjustment 37 to39 are independently rotated.

In addition, this invention concerning a scanning optical device is notlimited to the above-mentioned example of the embodiment. For example,it is possible for the scanning optical device to have a structure asfollows:

(A) The contact member is not limited to a screw for adjustment, and thenumber of the contact members is not limited to three. If the number ofthe contact members is increased, generally speaking, the range ofadjustment such as correction is broadened, and also the precision ofadjustment is improved.

(B) It is not required that all of the contact members are capable ofmoving in the upward and downward direction. For example, in a structurewherein the screws for adjustment 36 and 37 in the above-mentionedexample of the embodiment is not capable of moving, the projections 19and 20 are used as the contact members at this portion and the screwsfor adjustment 36 and 37 are not provided.

(C) The urging means is not limited to a leaf spring, and the number isnot limited to two. In a structure wherein an elastic member such as arubber is pressed, it is possible that the whole upper surface of thesecond cylindrical lens 6 is pressed by the elastic member.

(D) If it is desired that play for the screws for adjustment 37 to 39 inthe axial direction against the base 1 is eliminated, it may beappropriate to make a structure such that a coil spring or the like isdisposed between the head portion of the screws 37 to 39 and the base 1,in order that the screws for adjustment 37 to 39 may be always urged tothe downward direction.

(E) The fixing of the optical parts such as the second cylindrical lens6 is not limited to adhesive bonding.

(F) Although the correction for the scan lines is done by only thesecond cylindrical lens 6 in the above-mentioned example of theembodiment, it is possible to provide a mechanism for adjusting anddetermining the position in both of the second cylindrical lens 6 andthe fθ lens 5 respectively.

(G) It is also possible to carry out the adjustment and determination ofthe position of the second cylindrical lens 6 automatically. In thiscase, as a matter of course, the signal from the sensor on the surfaceto be scanned is fed back to the control portion, and the amount ofscrewing of the screws for adjustment 37 to 39 is adjusted by thecontrol portion in order that actual scan lines may become closer to theideal scan line.

THE SECOND EXAMPLE OF THE EMBODIMENT

FIG. 9 is a drawing showing the structure of an example of theembodiment of this invention concerning an image forming apparatus. Inan image forming apparatus in this example of the embodiment, formationof a color image using toners of four colors Y, M, C, and K with fourscanning optical devices arranged in the direction of sub-scanning.

In FIG. 9, the belt-shaped image bearing member 100 is driven in theclockwise direction (direction of the arrow marks), being entrainedaround the rollers 101 to 109. In the neighborhood of the image bearingmember 100, there are arranged the scanning optical devices 111 to 114in the direction of sub-scanning in such a manner as to face toward theimage bearing member 100. Each of the scanning optical devices 111 to114 has the same structure as has been shown in the above-describedfirst example of the embodiment, that is, the structure comprising atleast a light source, a collimator lens into which a light beam havingbeen emitted from said light source enters, a deflector which deflectsthe light beam having passed said collimator lens to the direction ofmain scanning, and an image forming lens which focuses the light beamhaving passed said deflector on the surface to be scanned of the imagebearing member.

In the above, the scanning optical device 111 is one for forming alatent image for Y (yellow) by using a laser beam, the scanning opticaldevice 112 is one for forming a latent image for M (magenta) by using alaser beam, the scanning optical device 113 is one for forming a latentimage for C (cyan) by using a laser beam, and the scanning opticaldevice 113 is one for forming a latent image for K (black) by using alaser beam.

Each of the scanning optical devices 111 to 114 is linked to theneighboring devices after determining the position. This linking is doneby using a link member. To describe it concretely, as shown in FIG. 10,the link member 200 is fixed to the both side portions of the respectivescanning optical devices 111 to 114 by using the plural screws 201 in amanner capable of being taken off, and after that, as shown in FIG. 11,the position of the one of the neighboring scanning optical devices isdetermined in the state of floating off against the other, and then theupper end surface and the lower end surface of the respective linkmembers of the neighboring scanning optical devices are bonded to eachother by using an adhesive of the UV-setting type or the like.

In this example of the embodiment, it is used the structure wherein thescanning optical devices 113 to 111 is successively stacked on thescanning optical device 114 one after another. In addition, although thelink member 200 and the screws 201 is not necessary in the first exampleof the embodiment, they are shown in FIG. 1 to FIG. 3 for the purpose ofmaking it easy to understand the structure of linking of the scanningoptical devices in the second example of the embodiment.

In FIG. 9 again, at the respective front stages of the scanning opticaldevices 111, 112, 113, and 114, the charging portions 121, 122, 123, and124, which give the image bearing member 100 electrostatic charges forY, M, C, and K respectively, are provided; at the respective rear stagesof the scanning optical devices 111, 112, 113, and 114, there areprovided the developing units 131, 132, 133, and 134, which develop thelatent images formed by the respective scanning optical devices 111,112, 113, and 114 by using the developers for Y, M, C, and Krespectively.

The transfer paper 142 in the paper feeding portion 141 is conveyed outby the paper feeding roller 143, and is fed to the transfer portion 151by the transport roller pair 144 and the timing roller 145. Thistransfer portion 151 is composed of the transfer electrode 152 whichtransfers the toner image on the image bearing member 100 to thetransfer paper 142 by corona discharging, and the pick off electrode 153which picks off the transfer paper 142 from the image bearing member 100by alternate current discharging.

The fixing portion 161 is composed of the heat roller 162 and thepressing roller 163, and fuses the toner image to stick to the transferpaper. The transfer paper 142 after this fixing process is ejected ontoa receiving tray by the transport portion 171 at the rear stage of thefixing portion 161. Further, the residual toner particles remaining onthe image bearing member 100 after transfer are scraped off in thecleaning portion 191, and are received in the collecting box 192.

In this example of the embodiment, the scanning optical devices 111 to114 and the developing units 131 to 134, which are assigned to thecolors Y, M, C, and K respectively, are arranged for every color in thedirection of sub-scanning; the scanning optical devices for therespective colors ill to 114 are let to form the latent electrostaticimages for the respective colors in a manner such that all the colortoner images should be mutually superposed precisely, and the developingunits 131 to 134 are let to develop the latent images respectively. Thatis, after forming the toner image of Y by using the scanning opticaldevice 111 and the developing unit 131, the toner image of M is formedon the toner image of Y superposed by using the scanning optical device112 and the developing unit 132, and the toner image of C is formed onthem by using the scanning optical device 113 and the developing unit133, and lastly the toner image of K is formed superposed on them byusing the scanning optical device 114 and the developing unit 134; thesesteps complete the formation of color toner image, which is transferredto the transfer paper 142 at the transfer portion 151.

In the case where independent scanning optical devices are used for therespective colors as described in the above, it is necessary that eachof the scanning optical devices forms a latent image in a manner suchthat it is precisely superposed on the others. In other words, it isnecessary to let the respective scanning optical devices 111 to 114trace scan lines of the same characteristic (the first condition), andalso it is necessary to determine the positions of the respectivescanning optical devices 111 to 114 in order that the scan lines tracedby the respective scanning optical devices are superposed on one another(the second condition).

In this example of the embodiment, as for the scanning optical devices111 to 114, the scanning optical device shown in FIG. 1 to FIG. 3 (thefirst example of the embodiment) is employed; therefore, it is easy toadjust each of the scanning optical devices 111 to 114 in order thatthey may trace scan lines having the same characteristic, and the firstcondition can be satisfied. Especially, if the optical parts for thesecond cylindrical lens etc designed to be of the same shape andmaterial and obtained from the same manufacturing lot are used, thecharacteristics of the optical parts as the individual ones are made tobe the same; thus, because the only thing to do further is merely toadjust and determine the positions of the optical parts, it is easier tolet each of the scanning optical devices 111 to 114 trace scan lines ofthe same characteristic.

On the other hand, in order to satisfy the second condition, in thisexample of the embodiment, for the scanning optical devices 111 to 114of which the adjustment of the individual device has been completed andof which to the both side portions the link members 200 are fixedrespectively by using the plural screws 201 for each in a manner capableof being taken off, the positions are determined with one of theneighboring scanning optical devices let to be in the state of floatingoff against the other, and the upper end surface and the lower endsurface of the respective link members of the neighboring scanningoptical devices are bonded to each other by using an adhesive of theUV-setting type or the like.

As for the fixture for adjustment, in this example, it is used a fixturethat is equivalent to one made up of four fixtures stacked on oneanother with a predetermined spacing, each of them being used at thetime of adjustment for the first example of the embodiment. That is, itis used a fixture wherein four parallel ideal scan lines correspondingto the colors Y, M, C, and K respectively are assumed to be on thesurface to be scanned, and a plurality of sensors are disposed on eachof the ideal scan lines. Each of the sensors can detect to what degreean actual beam is deviated from the above-mentioned ideal scan line.

The operation of adjustment is carried out as follows. First, a beam isgenerated in the scanning optical device 114, and the position of thisis determined in order that the actual scan line by this beam issuperposed on the ideal scan line for the color K (the scanning opticaldevice 114 is attached to the fixture for adjustment).

Next, the scanning optical device 113 is supported in the state offloating off against the scanning optical device 114, and is let togenerate a beam; the position is determined in order that the actualscan line of the beam may be superposed on the ideal scan line for thecolor C, and the upper end surface and the lower end surface of therespective link members 200 of the scanning optical devices 114 and 113are bonded to each other by an adhesive of the UV-setting type.

Further, the scanning optical device 112 is supported in the state offloating off against the scanning optical device 113, and is let togenerate a beam; the position is determined in order that the actualscan line of the beam may be superposed on the ideal scan line for thecolor M, and the upper end surface and the lower end surface of therespective link members 200 of the scanning optical devices 113 and 112are bonded to each other by an adhesive of the UV-setting type.

Lastly, the scanning optical device 111 is supported in the state offloating off against the scanning optical device 112, and in the sameway, the position is determined in order that the actual scan line ofthe beam may be superposed on the ideal scan line for the color Y, andthe upper end surface and the lower end surface of the respective linkmembers 200 of the scanning optical devices 112 and 111 are bonded toeach other by an adhesive of the UV-setting type.

In this connection, as for the cause of the relative deviation of thepositions between the neighboring scanning optical devices, thefollowing will be cited:

(1) Deviation in the upward and downward direction with regard to thesurface to be scanned (in the direction of sub-scanning)

The scan lines also deviate in the upward and downward direction.

(2) Deviation in the left and right direction with regard to the surfaceto be scanned (in the direction of main scanning)

The scan lines also deviate in the left and right direction.

(3) Deviation in the direction perpendicular to the surface to bescanned

The length of the scan line is increased or decreased (lateralmagnification is varied).

(4) Tilt around a horizontal axis perpendicular to the surface to bescanned

The scan lines are tilted in the upward and downward direction.

(5) Rotation around an vertical axis parallel to the surface to bescanned

The pixel density (interval between the neighboring pixels) on the scanlines is made large at one end portion and is made small at the otherend portion.

The detection of the above-described (1) to (5) can be done by disposingthree sensors on the ideal scan line. For example, if three sensorswhich detect respectively the position of the beam spot at the startingpoint, at the ending point, and at the center of the beam to be tracedactually by the scanning optical device, the deviations (1) to (4) canbe easily detected from the position of the beam spot at the startingpoint and the ending point, and the deviation (5) can be detected byjudging to which side the beam spot at the center shifts, the startingpoint or the ending point.

In this example of the embodiment, because the scanning optical deviceto be determined for its position is supported in a state of floatingoff against the neighboring scanning optical device with an arm of anassembling robot or the like for carrying out the adjustment, thescanning optical device can be shifted in any direction. Therefore, theabove-described deviations of the position can be easily reduced on thebasis of the outputs of the plural sensors. Further, it is determinedhow high the precision of the adjustment and determination of theposition by to what degree the image quality is required for the imageforming apparatus.

By an experiment of the inventors, it has been confirmed that if thedeviation of the latent images for the colors Y, M, C, and K formed onthe surface to be scanned is not larger than 200 μm, or desirably notlarger than 120 μm, the image can be recognized as an image giving nofeeling of disorder.

It is possible to carry out the above-described linking of the scanningoptical devices in the state of being loaded in the image formingapparatus, however actually, it makes the operation far more simple tolink the plural scanning optical devices beforehand and then load theimage forming apparatus with the scanning optical devices in the linkedstate.

As is obvious from the foregoing explanation, in this example of theembodiment, the positions of the plural scanning optical devices 111 to114 to be arranged in the direction of sub-scanning are adjusted anddetermined with the devices made to be in the state of floating, and thelink members are mutually linked after the determination of thepositions. Owing to this, by once precisely adjusting and determiningthe positions of the scanning optical devices 111 to 114, the positionalrelationship among the scanning optical devices 111 to 114 will never bevaried after that, and the deterioration of image quality owing to thescanning optical devices 111 to 114 can be prevented.

Further, because the link members 200 are fixed to the scanning opticaldevices 111 to 114 in a manner capable of being taken off, in the casewhere the determination of the positions is proved to be notsatisfactory after the linking of the neighboring scanning opticaldevices, it is possible that the link members 200 are taken off from thescanning optical devices 111 to 114, with the state of mutual linking ofthe link members 200 let to remain as it is, and then, new link members200 are attached to the scanning optical devices 111 to 114, and thepositions are determined again, and the new link members 200 are fixedto one another. Owing to this, it has become possible to utilize againthe scanning optical devices 111 to 114.

Furthermore, because an adhesive of the UV-setting type is used forfixing the link members 200 to one another, it is possible that thedetermination of the positions is carried out with a suitable amount ofthe adhesive injected between the link members, and the fixing isperformed immediately after determining the positions by applying anultra-violet ray; hence, the inking operation is made easy.

Besides, the above-described method of linking of the scanning opticaldevices 111 to 114 is one wherein the scanning optical devices 111 to114 are successively linked one by one; however, it is possible to bondthese devices at the same time after the positions of all the scanningoptical devices 111 to 114 are determined. Further, in theabove-described example of the embodiment, the link members 200 arefixed to one another by an adhesive of the UV-setting type; however, asa matter of course, it is possible to use other types of adhesive, andalso it is possible to screw the link members or to weld them.Furthermore, because the deviation of the latent images for the colorsY, M, C, and K can be detected by utilizing the technology of what iscalled pattern recognition, it may be appropriate to detect thedeviation of the positions by this method.

In some cases, keeping the superposition of the scan lines sure can beperformed not only by the hardware-like method of adjusting anddetermining the positional relationship among the scanning opticaldevices 111 to 114, but also by a software-like adjustment method todevise how to drive the scanning optical devices. However, because itincreases the burden on the software, it is desirable to make adjustmentby a hardware-like method as far as possible.

As has been explained in the foregoing, in this invention concerning ascanning optical device, it is provided a mechanism for adjusting anddetermining the position of the image forming lens comprising contactmembers capable of being in contact with the image forming lensrespectively at the three points, that is, the both end portions of theimage forming lens in the main scanning direction and an intermediateportion between these both end portions and an urging means for urgingthe image forming lens to the contact members. The image forming lens ispositioned in the vicinity of the surface to be scanned, and is easy tobe subjected to bending owing to its long size. In spite of this,according to this invention for adjusting the position of fixing thisimage forming lens, the scanning beam is corrected easily and with agood efficiency. In addition to it, because the image forming lens is incontact with the contact members at the three points, namely, the bothend portions in the main scanning direction and an intermediate portionbetween the both end portions respectively at the time of adjusting theposition of the image forming lens, it is possible to let the imageforming lens take any position by displacing the points of contact.

In the above-described invention, the points of contact of the imageforming lens at its both end portions in the main scanning directionwith the contact member are positioned on a straight line which isapproximately parallel to the main scanning direction, and the point ofcontact at the intermediate portion is placed at a position deviatedfrom a position on the above-mentioned straight line in the directionperpendicular to the surface to be scanned. Owing to this, theadjustment of position of the image forming lens can be performed bymoving at least one of the contact members placed at the above-mentionedthree points of contact in the direction of sub-scanning.

It is desirable that the image forming lens is fixed to the base inorder to prevent the deviation of the position of the image forming lensafter the adjustment of the position of the image forming lens. In theabove-described invention, this fixing is made at a position in theneighborhood of the point of contact at the intermediate portion of theimage forming lens. Owing to this, the image forming lens is fixed onlyat the intermediate portion, which makes it possible for the imageforming lens to expand and contract freely in accordance with thevariation of temperature and humidity of the surrounding; thus the imageforming lens is never bent unnaturally and it can be avoided theunfavorable influence that a complex bending etc are produced in thescan lines.

According to this invention concerning an image forming apparatus, thepositions of a plurality of scanning optical devices arranged in thedirection of sub-scanning are adjusted and determined in the state offloating off against the others, and the linking members are mutuallylinked after determining the positions. Owing to this, by once preciselyadjusting and determining the positions of the scanning optical devices,the positional relationship among the scanning optical devices willnever be varied after that, and the deterioration of image quality owingto the scanning optical devices can be prevented.

Further, because the linking members are fixed to the respectivescanning optical devices in a manner capable of being taken off, in thecase where the determination of the positions is proved to be notsatisfactory after the linking of the neighboring scanning opticaldevices, it is possible that these link members are taken off from thescanning optical devices, with the state of mutual linking of the linkmembers let to remain as it is, and then new link members are attachedto the scanning optical devices, and the positions are determined again,and the new link members are fixed to one another. Owing to this, it hasbecome possible to utilize again the scanning optical devices.

In this invention concerning an image forming apparatus, as a scanningoptical device, the above-described one is used, and at the same time,the mutual linking of the neighboring optical devices is made in thesame manner as described in the above-described invention. Owing tothis, according to this invention, in the respective scanning opticaldevices, the scanning beam can be corrected easily and with a goodefficiency. Further, because the linking members are mutually linkedafter determining the positions, the positional relationship among thescanning optical devices is never varied, and the deterioration of imagequality owing to the scanning optical device can be prevented.Furthermore, reutilization of the scanning optical devices has becomepossible.

Because an adhesive of the UV-setting type is used for fixing theabove-described link members to one another, fixing can be madeimmediately after determining the positions, and the linking operationis made easy.

What is claimed is:
 1. A color image forming apparatus, comprising: afirst image forming device including a first scanning optical device toform a first image and a first developing device to develop the firstimage into a first color toner image; and a second image forming deviceincluding a second scanning optical device to form a second image and asecond developing device to develop the second image into a second colortoner image; each of the first and second optical devices comprising: alight source which emits a light beam; a deflector which deflects thelight beam so as to scan an image forming surface in a main scanningdirection with the light beam; a plurality of lenses provided betweenthe deflector and the image forming surface to focus the deflected lighton the image forming surface, wherein each of the plurality of lenseshas a length along the main scanning direction; and a lens attitudeadjusting device including a tilting device to rotate at least one lensof the plurality of lenses around an axis parallel to the length of theone lens to adjust the characteristic of a scan line image, and whereinat least one of the tilting devices rotates the one lens so that thecharacteristic of the first scan line image and the characteristic ofthe second scan line image substantially become the same.
 2. Theapparatus of claim 1, wherein the deflector includes a polygon mirror.3. The apparatus of claim 1, further comprising: third and fourth imageforming devices including respective scanning optical devices andrespective developing devices, wherein each of the scanning opticaldevices comprises the light source, the deflector, the plurality oflenses, and the lens attitude adjusting device.
 4. The apparatus ofclaim 3, further comprising: an adjustment device adjusting at least oneof the first, second, third and fourth scanning optical devices so thatthe scanned images formed by of the first, second, third and fourthscanning optical devices are superposed on a recording sheet.
 5. Theapparatus of claim 1, wherein one of the plurality of lenses is a fθlens.
 6. The apparatus of claim 1, wherein one lens of the plurality oflenses is a cylindrical lens.
 7. The apparatus of claim 1, wherein theplurality of lenses comprise a fθ lens and a cylindrical lens.
 8. Theapparatus of claim 7, wherein the tilting device rotates the cylindricallens around an axis parallel to the length of the cylindrical lens so asto scan a straight line on the image forming surface.
 9. The apparatusof claim 7, wherein the tilting device of each of the first scanningdevice and the second scanning device comprises two tilting mechanismsto rotate the fθ lens and the cylindrical lens around an axis parallelto the length of each lens respectively so that the deflected lightscans a straight line on the image forming surface.
 10. The apparatus ofclaim 1, wherein the one lens is positioned closest to the image formingsurface among the plurality of lenses.
 11. The apparatus of claim 1,wherein the lens attitude adjusting device comprises first, second andthird supporting members to rotate at least one of the lenses around theaxis parallel to the length of the one lens, wherein the first andsecond supporting members are located at both end portions of the onelens for supporting the one lens respectively, and the third supportingmember is located at middle portion of the one lens for supporting theone lens so that the supporting point of the third supporting member isdistant with respect to an imaginary line connecting the supportingpoints of the first and second supporting members.
 12. The apparatus ofclaim 1, wherein the lens attitude adjusting device comprises a screw.13. The apparatus of claim 1, wherein the characteristic of the image isa straight line.
 14. The apparatus of claim 1, wherein each of the firstand second images is a line image and the characteristic of the image isthe shape of the line image.
 15. The apparatus of claim 14, wherein thetilting device of each of the first and second optical devices rotatesthe respective one lens so as to make the respective line image tobecome a desired line.
 16. The apparatus of claim 15, wherein thetilting device of each of the first and second optical devices rotatesthe respective one lens so as to make deviations between the respectiveline image and the desired line smaller than 200 μm.
 17. The apparatusof claim 16, wherein the deviations are made smaller than 120 μm. 18.The apparatus of claim 14, wherein the tilting device of at least one ofthe first and second optical devices rotates the respective one lens soas to make the first and second images to conform to each other.
 19. Theapparatus of claim 1, further comprising: an adjustment device adjustingat least one of the first and second scanning optical devices so thatthe scanned images formed by the first and second image forming devicesare superposed on a recording sheet.
 20. The apparatus of claim 1,wherein the image forming surface is a surface of a common image bearingmember.
 21. A scanning optical system for use in a color image formingapparatus, comprising: first and second scanning optical devicesscanning in a main scanning direction with a light beam so that each ofthe devices forms an image on an image forming surface, each of thefirst and second scanning optical devices comprising: a light sourcewhich emits a light beam; a deflector which deflects the light beam inthe main scanning direction; a plurality of lenses, provided between thedeflector and the image forming surface, which focus the deflected lightbeam on the image forming surface, wherein each of the lenses has alength along the main scanning direction; and a tilting device rotatingat least one of the lenses around an axis parallel to the length of theone lens so as to adjust the characteristic of a scan line on the imageforming surface, and wherein at least one of the tilting device of thefirst scanning device and the tilting device of the second scanningdevice rotates the one lens so that the characteristic of the first scanline and the characteristic of the second scan line substantially becomethe same.
 22. The apparatus of claim 21, wherein the deflector includesa polygon mirror.
 23. The apparatus of claim 21, further comprising:third and fourth scanning optical devices, each of the third and fourthscanning optical devices comprising the light source, the deflector, theplurality of lenses, and the tilting device.
 24. The apparatus of claim23, further comprising: an adjustment device adjusting at least one ofthe first, second, third and fourth scanning optical devices so that thescanned images formed by the first, second, third and fourth scanningoptical devices are superposed on a recording sheet.
 25. The apparatusof claim 21, wherein one of the plurality of lenses is a fθ lens. 26.The apparatus of claim 21, wherein one of the plurality of lenses is acylindrical lens.
 27. The apparatus of claim 21, wherein the pluralityof lenses comprise a fθ lens and a cylindrical lens.
 28. The apparatusof claim 27, wherein the tilting device of each of the first scanningdevice and the second scanning device rotates the cylindrical lensaround an axis parallel to the length of the cylindrical lens so as toscan the straight line on the image forming surface.
 29. The apparatusof claim 27, wherein the tilting device of each of the first scanningdevice and the second scanning device comprises two tilting mechanismsto rotate the fθ lens and the cylindrical lens around an axis parallelto the length of each lens respectively so that deflected light scans astraight line on the image forming surface.
 30. The apparatus of claim21, wherein the one lens is positioned closest to the image formingsurface among the plurality of lenses.
 31. The apparatus of claim 21,wherein the tilting device comprises first, second and third supportingmembers to rotate at least one of the lenses around the axis parallel tothe length of the lens, wherein the first and second supporting membersare located at both end portions of the lens for supporting the lensrespectively, and the third supporting member is located at middleportion of the lens for supporting the lens so that the supporting pointof the third supporting member is distant with respect to an imaginaryline connecting the supporting points of the first and second supportingmembers.
 32. The apparatus of claim 21, wherein the tilting devicecomprises a screw.
 33. The apparatus of claim 21, wherein thecharacteristic of the scan line is a straight line.
 34. The apparatus ofclaim 21, further comprising: an adjustment device adjusting at leastone of the first and second scanning optical devices so that the scannedimages formed by the first and second scanning optical devices aresuperposed on a recording sheet.
 35. The apparatus of claim 21, whereinthe image forming surface is a surface of a common image bearing member.36. The apparatus of claim 21, wherein the tilting device of each of thefirst and second optical devices rotates the respective one lens so asto make deviations between the respective line image and a desired linesmaller than 200 μm.
 37. The apparatus of claim 36, wherein thedeviations are made smaller than 120 μm.
 38. A color image formingapparatus including a developing device for color toner image,comprising: first and second scanning optical devices scanning in a mainscanning direction with a light beam so that each of the devices form animage on an image forming surface, each of the first and second scanningoptical devices comprising: a light source which emits a light beam; adeflector which deflects the light beam in the main scanning direction;a plurality of lenses, provided between the deflector and the imageforming surface, which focus the deflected light beam on the imageforming surface, wherein each of the lenses has a length along the mainscanning direction; and a tilting device rotating at least one of thelenses around an axis parallel to the length of the lens so as to adjustthe characteristic of a scan line on the image forming surface, andwherein at least one of the tilting device of the first scanning deviceand the tilting device of the second scanning device rotates the lens sothat the characteristic of the first scan line and the characteristic ofthe second scan line substantially become the same.
 39. The apparatus ofclaim 38, wherein the deflector includes a polygon mirror.
 40. Theapparatus of claim 38, further comprising: third and fourth scanningoptical devices, each of the third and fourth scanning optical devicescomprising the light source, the deflector, the plurality of lenses, andthe tilting device.
 41. The apparatus of claim 40, further comprising:an adjustment device adjusting at least one of the first, second, thirdand fourth scanning optical devices so that the scanned images formed bythe first, second, third and fourth scanning optical devices aresuperposed on the recording sheet.
 42. The apparatus of claim 38,wherein one of the plurality of lenses is a fθ lens.
 43. The apparatusof claim 38, wherein one of the plurality of lenses is a cylindricallens.
 44. The apparatus of claim 38, wherein the plurality of lensescomprise a fθ lens and a cylindrical lens.
 45. The apparatus of claim44, wherein the tilting device rotates the cylindrical lens around anaxis parallel to the length of the lens so as to scan a straight line onthe image forming surface.
 46. The apparatus of claim 44, wherein thetilting device of each of the first scanning device and the secondscanning device comprises two tilting mechanisms to rotate the fθ lensand the cylindrical lens around an axis parallel to the length of eachlens so as to scan a straight line on the image forming surface.
 47. Theapparatus of claim 38, wherein the one lens is positioned closest to theimage forming surface among the plurality of lenses.
 48. The apparatusof claim 38, wherein the tilting device comprises first, second andthird supporting member to rotate at least one of the lenses around theaxis parallel to the length of the lens, wherein the first and secondsupporting members are located at both end portions of the lens forsupporting the lens respectively, and the third supporting member islocated at middle portion of the lens for supporting the lens so thatthe supporting point of the third supporting member is distant withrespect to an imaginary line connecting the supporting points of thefirst and second supporting members.
 49. The apparatus of claim 38,wherein the tilting device comprises a screw.
 50. The apparatus of claim38, wherein the characteristic of the scan line is a straight line. 51.The apparatus of claim 38, further comprising: an adjustment deviceadjusting at least one of the first and second scanning optical devicesso that the scanned images formed by the first and second scanningoptical devices are superposed on the recording sheet.
 52. The apparatusof claim 38, wherein the image forming surface is a surface of a commonimage bearing member.
 53. The apparatus of claim 38, wherein the tiltingdevice of each of the first and second optical devices rotates therespective one lens so as to make deviations between the respective lineimage and a desired line image smaller than 200 μm.
 54. The apparatus ofclaim 53, wherein the deviations are made smaller than 120 μm.